For the past seven years, they’ve been going underground to locate, map and seal off the tunnels used by cartels to smuggle drugs from Mexico to San Diego and beyond.

Theirs is a little-known part of the high-stakes hide-and-seek game that plays out daily along the border. While much of the attention, especially lately, has been focused on walls and what happens above ground, more than 80 tunnels have been found in California and Arizona since 2011.

Some have been almost 3,000 feet long and contain tracks for motorized carts, as well as lights, elevators and ventilation. One ended underneath a house in Calexico built just to provide cover for the tunnelers.

San Diego is a hotbed for a lot of this. Warehouses constructed close to the border in Otay Mesa and Tijuana provide camouflage: an out-of-view place for a tunnel to start and another for it to end.

It’s also where the clay soil is especially good for this kind of thing — not as soft and collapse-likely as it is to the west, and not as rocky and hard as it is to the east.

“This,” said Lance LeNoir, gesturing at the warehouses and the ground between them, “is what makes San Diego grand central for the long, sophisticated tunnels.”

LeNoir is an operations officer for the Border Patrol. He heads the five-member Tunnel Rats, and he was standing one recent weekday morning near what’s known in law-enforcement circles as the Galvez Tunnel.

Discovered in December 2009, it stretches 762 feet from a warehouse in Tijuana toward a warehouse on the U.S. side, just west of the Otay Mesa Port of Entry.

The tunnel is 6 feet tall by 4 feet wide, large by tunneling standards, and 100 feet below the surface in some spots, sloped to allow groundwater to flow out of the way.

The traffickers had been working on it for about 18 months and had not yet finished when it was discovered after a tip from an informant. A dozen people were arrested inside.

Now what’s left of the tunnel, about 30 feet, is used for training by the Tunnel Rats. They practice rescues and test their equipment there.

It’s where they take government officials and the media when they want to show the kind of subterranean activity they are up against.

During a recent visit, LeNoir was asked whether he believed, at that moment, someone somewhere was digging a tunnel.

“Of course they are,” he said. “Of course.”

A Nod to Vietnam

The Tunnel Rats borrow their name from the Vietnam War forces who went underground in search of enemy fighters, sometimes engaging in hand-to-hand combat.

“They had it a lot tougher than we do,” LeNoir said. “We use the name in homage to them.”

They wear T-shirts with “Tunnel Rat” on the back, above a drawing of a fierce-looking rodent carrying a gun in one hand and a flashlight in the other. Below the drawing is a Latin phrase, also from Vietnam, that translates into “Not worth a rat’s a--.”

Several of the team members are military veterans, although none is old enough to have served in Vietnam, and their uniforms resemble those worn by soldiers: camouflage pants, helmets, vests, guns.

Team members volunteer for the assignment, and to join they first have to crawl through a two-foot wide pipe for about 20 yards. That helps weed out agents who are claustrophobic and maybe don’t know it, and it also gets them ready for what they’ll face in the field.

Increasingly, the tunnels are getting narrower and shorter — quicker to build that way, and cheaper. One found last year was only 14 inches wide.

Getting inside the Galvez Tunnel is simple by comparison. Visitors climb down 70 feet of metal ladders, installed in a concrete shaft built after the underground smuggling route was discovered. It intersects the tunnel in a spot located between the primary and secondary border fences.

The air feels heavy at the bottom, and warm. Overhead lights illuminate the sides of the tunnel, which still bear the tool marks of those who built it.

Galvez gets its name from a street in Tijuana that runs next to the warehouse where the tunnel originated. It’s considered “sophisticated” because of its length and some of the things found inside it.

But “sophisticated” is a relative term.

“These tunnels wouldn’t meet any mining or construction standards that we are familiar with,” LeNoir said. If wood is found inside shoring up the walls and roof, it’s not because of a devotion to structural integrity, he said, but because a collapse happened while they were working and they had to fix it

“When you see 2-by-4s attached to plywood with drywall screws, you know you’re not looking at something that’s been carefully engineered,” he said.

Here’s what team members sometimes call the tunnels: “Holes in the ground at significant depth.”

What does impress them, though, is the persistence of the tunnelers, who aren’t always there by choice, conscripted at gunpoint by the cartels. Impressed by the workload. (Multiple eight-hour shifts, sometimes all day, using power drills, picks and shovels. They eat and sleep on site.) Impressed by the dirt removal. (It’s put it in sandbags and stored in the warehouses, or if there’s an empty room, just piled there.)

“They’re willing to dig and dig and dig without really knowing where they’re going to end up,” LeNoir said. “You have to respect their imagination and their audacity.”

Deja Vu

In our high-tech age, people sometimes think finding tunnels should be easy. Just stick motion-detectors in the ground, they say. Just use ground-penetrating radar.

It’s not that simple. Many such devices are susceptible to interference from passing cars and trucks and from underground power lines. They’re set off inadvertently by animals or the wind.

Still, the hunt for a silver bullet continues. The eight border wall prototypes recently built in Otay Mesa are being tested now for their ability to, among other things, deter tunneling. Each is supposed to include sensors that will detect someone approaching the wall or trying to breach it.

Until that kind of solution arrives, investigators usually find tunnels the old-fashioned way. They patrol the border. They talk to warehouse owners and occupants and ask them to report anything unusual or suspicious.

The Tunnel Rats are part of the Drug Tunnel Task Force, which also includes representatives from Homeland Security, the Drug Enforcement Agency and Immigration and Customs Enforcement. It was formed in 2003 as officials noticed that even though most drugs are driven across the border at ports of entry, hidden inside cargo trucks and other vehicles, tunnels were becoming a major player.

At the Calexico one — the first time traffickers are known to have purchased land and built a house on it to conceal a tunnel — agents found more than a ton of marijuana. That was a small find: Other tunnels have led them to caches of more than 20 tons.

Originally, the underground team was focused on smugglers who used existing storm drains and sewer systems to move people across the border illegally. As more and more cross-border tunnels were discovered — 13 in the San Diego sector alone in 2006 — the team began focusing on that. They developed skills in geology, air monitoring and emergency extractions.

After a tunnel is found and cleared of smugglers, the Tunnel Rats are called in to check it for evidence and map it. They make sure the air is safe and the ground stable, and then crawl in with tape measures, compasses and lasers.

Then concrete is poured into the tunnels at various places on the U.S. side — “remediation” that has cost the federal government about $10 million since 2007.

Team members said what they like most about the work is the variety. “Every tunnel is different,” several of them said.

Their work ebbs and flows from year to year. Through the end of August, seven tunnels — three operational and four not yet finished — had been discovered in the fiscal year that started Oct. 1, 2016, according to the Border Patrol. In the eights weeks so far this year: zero.

Over the past 10 years, the number of tunnels discovered has fluctuated between one and nine.

Sometimes the work has a feeling of deja vu. Officials on the Mexican side of the border don’t always have the resources to seal tunnels there.

At least eight times in recent years, the Border Patrol says, newly discovered tunnels turned out to be old ones. The smugglers started in Mexico using what was already there and when they came to the concrete on the U.S. side, they dug around it.

Until they were found again, another round of hide-and-seek that shows no signs of ending.